feat: Phase 4 — MapLibre route map + SVG elevation chart on activity screen

- Add /api/activity/{id}/geojson and /api/activity/{id}/timeseries endpoints
  (bearer-token-gated, falls back from _merged to raw activities dir)
- Rewrite activity detail screen with MapLibreGL v11 API (Map, Camera,
  GeoJSONSource, Layer) and react-native-svg area chart with gradient fill
- On-demand fetch for remote activities that have no local geojson/timeseries
- Add react-native-svg dependency; requires dev build (npx expo run:android)

mobile/package.json

@@ -27,6 +27,7 @@
     "react-native": "0.81.5",
     "react-native-safe-area-context": "~5.6.0",
     "react-native-screens": "~4.16.0",
+    "react-native-svg": "~15.8.0",
     "react-native-webview": "13.15.0",
     "@maplibre/maplibre-react-native": "~11.0.0"
   },